It is recognized in the art that rapid freezing of a water-bearing substance will promote the distribution of a great number of small ice crystals and slow freezing provides a lesser number of larger ice crystals. As applied to freeze dried coffee, it is known that a rapidly frozen coffee extract containing the smaller ice crystals yields a light-colored, tan soluble coffee whereas a slowly frozen extract containing larger ice crystals provides a darker coffee which more nearly resembles the appearance of roast and ground coffee.
The art establishes several methods of freezing coffee extract slowly enough to provide a darker-colored freeze dried coffee product. A method of freezing coffee extract on a chilled metal belt is disclosed in U.S. Pat. No. 3,253,420 to De George wherein freezing takes place over an approximately 15 minute period. A method of freezing whereby coffee extract is agitated while being chilled from its ice point to a temperature below its eutectic point over a period of between 15 minutes and 30 minutes is disclosed in U.S. Pat. No. 3,399,061 to Lutz. While these disclosures provide for a dark-colored freeze dried coffee, they are of necessity slow and hence limiting on productivity. A process aimed at improving productivity, described in U.S. Pat. No. 3,373,042 to Elerath et al., involves first slushing the coffee extract prior to the rapid freezing of said extract. The process makes no provision for promoting ice crystal growth by subsequently melting some slush and so does not serve to darken color.
More rapid methods of freezing coffees extracts which do darken finished product color are known but they are not without certain drawbacks. For instance, U.S. Pat. No. 3,443,963 to Simon et al. describes a process wherein coffee extract is first chilled to about -5.degree. C. and held at that temperature for nearly 20 minutes prior to rapid freezing. Though maintenance of the extract at -5.degree. C. will promote some crystal growth, the process does not provide for a melting step wherein the smaller ice crystals are destroyed.
Another disclosure, U.S. Pat. No. 3,966,979 to Katz et al., describes a multi-layered freezing process wherein the upper portion of a fast frozen extract layer is melted by having a layer of warm coffee extract placed on top and both layers are then frozen prior to the addition of the next layer. The melting and freezing of the upper portion of a layer promotes larger ice crystals therein but the lower portion of the fast frozen layer is unaffected which lower portion presumably contains the smaller ice crystals developed by rapid freezing.
It is an object of the present invention to provide a process for producing a dark-colored freeze dried coffee. It is a further object of this invention to provide a multi-step freezing process that is not limiting on productivity.